Francois said that the Pratts apparently heard noises outside the yacht, when it was in the southern Vieux Fort harbour area, and were attacked when they went to investigate.

"[The attackers] intention it would appear was to commit a robbery because they asked them for cash and other items," he said.

"They were not able to produce it so that resulted in, I guess, some interaction between the Pratts and the individuals.

"Margaret Pratt went in search of her husband who was not on the yacht at the time and he was found floating in the water nearby. She called for some assistance and then they were taken to the St Jude hospital where the gentleman was pronounced dead."

Authorities said five suspects were in custody and would be charged at a later date, but did not specify when.

The couple, from Warwickshire in England, had been travelling aboard their yacht, the Magnetic Attraction, since June to celebrate Margaret Pratt's 60th birthday.

St Lucia, an island of about 163-thousand people, has seen an increase in violent crimes that authorities blame on drug trafficking.

Police, however, reported 34 killings last year – the lowest in four years.